In the field of Liquid Chromatography, an unknown substance is introduced along with a solvent solution into a column. The solvent solution is usually comprised of one or more liquids which are mixed together in known proportions. However, during testing of a particular sample, it has been found desirable to change the proportions of the individual liquids which comprise the solvent solution into which the sample is introduced. As a result, solvent gradients are useful, inter alia, for optimizing separation time or for choosing an isocratic composition for an unfamiliar separation. Generally, if an isocratic separation can be performed in a reasonable time period, there is no need to implement a gradient. However, if a separation involves extensive time, such as for example 30 minutes, the use of a gradient may be useful. If the earliest peaks of a chromatogram are unresolved and the later peaks are eluted slowly, either a gradient or a different column/mobile phase combination is necessary.
In many instances, liquid chromatography systems are designed to accommodate only a single solvent, i.e. to perform isocratic separations. However, two such systems can be coupled, via a control mechanism to perform linear solvent gradient analysis. Heretofore, conventional control mechanism relied upon either analog circuits. As a result, the accuracy of such conventional control mechanisms was susceptible to temperature changes and signal drift. Hence, the output signal to the pumps, being controlled, varied and was frequently difficult to reproduce.
A related reference in this field is U.S. Pat. No. 4,084,246 issued Apr. 11, 1978, which is directed to a generator for controlling the flow rate of two solvents where it is desired to maintain the total flow of the two solvents constant while permitting precise control of the percent of one solvent in the mixture.
While the above prior art systems have met with success for their intended purposes, the present invention is directed to improvements thereover, particularly with respect to certain installations, as will become apparent as the description proceeds.